The present invention is directed to a display device in which a pair of plates are interconnected in space relationship to form a cavity for receiving a medium, such as a liquid crystal, that has switchable optical states. For each digit place, one plate will have electrical conducting coatings arranged as an electrode in a pattern of seven segments that are interconnected to form four groups of actuatable segments and the second plate has a pair of electrodes arranged to overlie the segments of the groups.
A display in which the seven segments on one plate are interconnected in groups and the back electrode is divided into two parts overlying a segment of each group is disclosed in U.S. Pat. No. 3,781,863, which was based on the same Japanese patent application as German O.S. No. 21 63 634. In operation of this device or other known liquid crystal displays, a failure of one of the segments or electrodes to become actuated at the desired time due to an interruption or break in the supply circuit or due to the shorting of the two segments together will cause an erroneous display to be exhibited. If a different number or symbol appears instead of the proper symbol, then a falsification cannot be recognized by the operator. In calibrated instruments, such a circumstance is not acceptable.
If the display device generates its own light, then the necessary current strengths are generally so high that one can monitor the display by means of a measurement of the segmented currents. This method however is not practical in displays such as the liquid crystal display, which modulates external light and use a very small amount of energy. Therefore up until now, the liquid crystal display usually had either received redundant or even finer segmentation in order to prevent the display of erroneous information. Examples of these devices are disclosed in U.S. Pat. No. 4,037,927 which includes the disclosure of the German O.S. No. 24 02 749 and in U.S. Pat. No. 4,054,368 which includes the disclosure of German O.S. No. 24 59 488. A difficulty with this precaution is that the finer segmentation increases the cost the fabrication of the device and and increases both the cost and number of problems with controlling the device.